A general example of the suppression countermeasure against EMI noise is explained with reference to FIG. 8. Note that in FIG. 8, a case of communication devices, such as a switch, an IP (Internet Protocol) switch, and a router, is shown.
A station power supply or a commercial power supply 1 is supplied to a product and device 3, which is a communication device, via an external feed cable 2. In the product and device 3, supply power is supplied to a power supply circuit 6 via a filter circuit 5, and further, supplied to an electronic circuit board (printed circuit board) 4 as an operation power supply of an electronic circuit 12, which is mounted on the electronic circuit board (printed circuit board) 4, via an internal feed line 7 and an internal feed connector 8.
A plurality of electronic circuits 12, an onboard power supply 11, and a filter circuit 9 are mounted on this electronic circuit board 4.
Usually, a power supply voltage supplied via the power supply circuit 6 from the station power supply or the commercial power supply 1 is different from a voltage required by the electronic circuit 12, and referred to as a primary power supply. This primary power supply is made to be a voltage necessary for an operation of the electronic circuit 12 by the onboard power supply 11, and is supplied to the electronic circuit 12. The power supply supplied to the electronic circuit 12 is referred to as a secondary power supply here.
The primary power supply is supplied to an input unit of the onboard power supply 11 via a primary side feed line 10 from the internal feed connector 8, and the secondary power supply is supplied to each electronic circuit 12 via a secondary side feed line 13 from an output unit of the onboard power supply 11.
Generally, the primary power supply and the secondary power supply are electrically separated, so that high frequency noise 102a by the operation of the electronic circuit supplied by the secondary power supply does not propagate to the primary power supply.
However, by electromagnetic coupling of the primary side power supply wire 10 and the secondary side power supply wire 13 and coupling inside the onboard power supply 11, the high frequency noise 102a generated in the electronic circuit 12 enters the primary power supply (indicated by high frequency noise 102b and 102c), and EMI noise (EMI radiation) 101 is generated via the internal feed line 7 and the external feed cable 2. Therefore, the filter circuit 9 such as a capacitor and a common mode choke coil is inserted along the primary side feed line 10, and this high frequency noise is suppressed.
Note that as an example of such filter circuit using an impedance element, there are PTL 1 and 2.
Moreover, with reference to PTL 3, a technique is disclosed that by enlarging an area of an overlapping part (a plurality of parts) of a power supply wire disposed in a first layer and a reference potential (ground: GND) wire disposed in a second layer that compose the feed line on the circuit board, an apparent capacity is attempted to increase and the EMI noise is suppressed.